It has been known since the 1970's that overheating of the voice coil of electro dynamic transducers is one of the main causes of failure of such transducers. Therefore, transducer manufacturers normally specify a maximum power rating, which they can guarantee that the transducer will survive being exposed to for an extended period of time. However, music is very dynamic, which means that such a static limit is far from optimal, because the transducers can handle much more power than the maximum power rating for shorter periods of time. Therefore different systems for thermal protection of transducers have been researched since the late 1970's. With such a system it would not be possible to damage the transducer even though more power than the maximum rated power is applied for shorter periods of time. However, as of today such protection systems have not seen widespread commercial use.
Previous thermal protection systems can be divided into three categories:
1) Feed forward protection systems, where the voice coil temperature is predicted using a thermal transducer model. The signal is then attenuated based on the predicted temperature. Such systems are very susceptible to modeling inaccuracies, as well as the fact that the ambient temperature is a very important parameter for the temperature prediction model. This means that either a given ambient temperature is assumed, which can be inaccurate, or an ambient temperature sensor is required, which increases the complexity and adds cost.
2) Feedback systems, where the temperature is measured in one way or another. Typically, since the voice coil is moving, it is difficult to attach a temperature sensor directly to the voice coil. Therefore, in such systems, the temperature of the transducer magnet can be measured instead and then a thermal model can be used to predict the voice coil temperature based on the magnet temperature. The signal is then attenuated based on the measured or predicted temperature. The disadvantages of such a system are first, the addition of a temperature sensor, which increases the overall system cost. Second the system is sensitive to modeling inaccuracies.
3) Systems where a DC component is added to the audio signal. The DC-resistance of the voice coil can then be calculated by low pass filtering the signal and using Ohms law to calculate the DC-resistance. Since there is a linear relationship between temperature and resistance of an electrically conducting material, the voice coil temperature can be calculated based on its DC-resistance. The signal is then attenuated based on the calculated temperature. The disadvantages of this type of system are that the constant DC in the signal will produce a slight offset of the transducer diaphragm, which decreases the linearity of the transducer. Furthermore the constant DC will dissipate power in the voice coil, increasing idle power consumption of the system and heating up the voice coil unnecessarily. There is also the technical problem that most power amplifiers are AC-coupled by nature, which means that they are incapable of reproducing DC signals.
US 2004/0178852 discloses a system where voltage across the voice coil, and current through the voice coil are measured, and used to calculate the DC resistance of the voice coil. The resistance is then used to determine the temperature, so as to enable attenuation of the input.